The present invention generally relates to rotary recording mediums having a guide track and recording and reproducing apparatuses therefor, and more particularly to a rotary recording medium having a guide track which is used for guiding a light beam when recording and reproducing an information signal by the light beam, and a recording and reproducing apparatus for recording and reproducing the information signal on and from the rotary recording medium by accurately controlling the tracking of the light beam.
When recording an information signal on a rotary recording medium (hereinafter simply referred to as a disc) by a light beam, it is known from a Japanese laid-open patent application No. 49-113601, for example, to first form a guide track on an unrecorded disc and then record the information signal on the disc by guiding the light beam by use of the guide track. The guide track may be constituted by a pair of continuous spiral tracks having a predetermined depth and located on both sides of a main track on and from which the information is recorded and reproduced. On the other hand, the guide track may be constituted by a single continuous spiral track having a predetermined depth.
When recording or reproducing the information signal on or from the disc which is formed with the pair of continuous spiral guide tracks, a pair of sub light beams for tracking are used in addition to a main light beam for recording or reproducing the information signal. The sub light beams are irradiated on the disc at positions which respectively lead and trail a position irradiated by the main light beam in the scanning direction and are mutually deviated in the width direction of the main track. A tracking control is performed so that one of the sub light beams is irradiated on a portion of one of the guide tracks and the other of the sub light beams is irradiated on a portion of the other of the guide tracks. Accordingly, the tracking of the main light beam is controlled to accurately scan the main track so as to record or reproduce the information signal on or from the main track.
On the other hand, in the case of the disc which is formed with a continuous spiral groove having a depth .lambda./8 as the single continuous spiral guide track, a single light beam is used to record and reproduce the information signal on and from the guide track, where .lambda. denotes the wavelength of the light beam. At the time of the recording, the light beam scans the guide track to form pits of the information signal within the guide track, and the tracking of the light beam is controlled by use of first (-1-st and +1-st) diffracted lights which are generated when the light beam scans the guide track having the depth .lambda./8. In other words, when the light beam is irradiated on the guide track at a center position thereof, the light intensities of the first diffracted lights on right and left sides of the center position are equal to each other. However, when the light beam is irradiated at a position deviated from the center position of the guide track, a difference is introduced between the light intensities of the first diffracted lights. Hence, the tracking control is performed by detecting the difference between the first diffracted lights.
However, in the case of the conventional disc which is formed with the pair of continuous spiral guide tracks, conditions of the leading and trailing sub light beams differ when recording the information signal by use of the main light beam. That is, the trailing sub light beam is affected by the pits of the information signal recorded by the main light beam. For this reason, there is a problem in that it is impossible to obtain a correct tracking error signal.
On the other hand, in the case of the conventional disc which is formed with the single continuous spiral guide track, the first diffracted lights may or may not be generated when the light beam traverses the guide track at the time of the reproduction, depending on whether the light beam is irradiated on the groove portion or the non-groove portion of the disc. In some cases, reflected lights irradiated on a four-element photodetector which is divided into four optical sensors may become asymmetrical even when the light beam is correctly focused on the disc, and a focal servo operation may be performed erroneously. The reflected lights from the disc are irradiated on the four-element photodetector via a cylindrical lens. A light receiving surface (that is, the irradiated pattern) on the four-element photodetector becomes a perfect circular pattern when the focal point of the light beam is correctly on the disc (that is, in focus), and the irradiated pattern becomes an oval pattern when the focal point of the light beam is above or below the disc (that is, out of focus). Hence, there is a known astigmatic focusing method in which a subtraction is performed between a sum of outputs of a first pair of diagonally arranged optical sensors and a sum of outputs of a second pair of diagonally arranged optical sensors so as to obtain an error signal, and the error signal is supplied to a focal system so as to correct the focal error.
According to a recording and reproducing apparatus which employs the astigmatic focusing method to detect the focal error, the irradiated pattern on the four-element photodetector becomes asymmetrical and the sum of the outputs of one pair of diagonally arranged optical sensors becomes greater than the sum of the other pair of diagonally arranged optical sensors as the light beam traverses the guide track of the disc, even when the light beam is correctly focused on the disc. Hence, it is inappropriate to employ the astigmatic focusing method of detecting the focal error in the recording and reproducing apparatus for playing the disc which is formed with the single guide track. For this reason, in the recording and reproducing apparatus for playing the disc which is formed with the single guide track, it is desirable to employ a knife edge method of obtaining the focal error signal by placing a knife edge at a point where the reflected light beam becomes narrowest in the focused state and by detecting the reflected beam in a two-element photodetector which comprises two optical sensors. However, according to the knife edge method, there is a problem in that the optical system becomes complex because of the high accuracy required for the knife edge, two-element photodetector, lens, focal distance and the like.